Cables for high-voltage electronic devices such as CT devices for medical use and X-ray devices to which a high DC voltage is applied are required (i) to be small in outside diameter and light-weighted, (ii) to have good flexibility and be resistant against movement and bending, (iii) to be small in capacitance and be capable of following the repeated application of high-voltages, and (iv) to have heat resistance high enough to endure the heat generation of an X-ray vacuum tube part.
As such a cable for high-voltage electronic devices (for example, an X-ray cable), there has been known one in which two low-voltage cable cores and one bare conductor or two are twisted together, an inner semiconductive layer is provided thereon, and a high-voltage insulator, an outer semiconductive layer, a shielding layer, and a sheath are further provided thereon in the order mentioned. As the high-voltage insulator, used is a composition with its base being EP rubber (ethylene propylene rubber) that is light-weighted and flexible and has relatively good electric characteristics (see, for example, Reference 1).
In recent years, EP rubber compositions having a low dielectric constant (about 2.3) have been put into practical use and cables for high-voltage electronic devices using this as a material of the high-voltage insulator and having a smaller diameter and smaller capacitance have been developed.
These EP rubber compositions, however, have a problem that their withstand voltage characteristic is not high enough because their volume resistivity greatly lowers as temperature increases due to high temperature dependence of the volume resistivity. Specifically, in the aforesaid cable, when the temperature of the conductor increases due to energization, the temperature of the high-voltage insulator nearby increases, but because the EP rubber composition whose electric resistivity has high temperature dependence is used as the high-voltage insulator, the volume resistivity of the high-voltage insulator near the conductor lowers. As a result, an electric field concentrates near an interface between the outer semiconductive layer and the high-voltage insulator, which tends to cause dielectric breakdown. This phenomenon also occurs in an AC power cable, but causes a great problem especially in a DC power cable such as a cable for high-voltage electronic devices. This phenomenon causes a still greater problem in a cable realizing a diameter reduction by the use of the low-dielectric constant EP rubber composition because its high-voltage insulator is thin. Therefore, there is a demand for an insulating material whose volume resistivity has low temperature dependence.